1. Undulator Physics Considerations and Commissioning Heinz-Dieter Nuhn, SLAC / LCLS May 10, 2005
Final Break Length Choice
AC Conductivity Wakefields
Undulator Tolerance Budget
Cradle Component Arrangement and Alignment
Commissioning Plan
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2. Undulator Break Lengths (Old Strategy) New Strategy
Characteristic Lengths
Length of Undulator Strongback (Segment): Lseg = 3.4 m
Distance for 113 x 2p Phase Slippage: L0 = (3.668 m) m
Distance for 2p Phase Slippage in Field Free Space: Linc = lu (1+K2/2) = m
Standard Break Lengths Used
Use parameter n to characterize different phase length choices Ln = L0 - Lseg +(n-1)Linc
Use 2 Short Breaks Followed by 1 Long Break in n-Pattern 2 – 2 – 4 ([0.482 m – m – m]) [0.470 m – m – m]
Fine Tuning of Initial Break Length
Based on Simulations using Linear Simulation Code, RON
Small length increases for first 3 break lengths [0.045 m – m – m]
Total Undulator Length (from beginning of strongback 1 – end of strongback 33):
Lund = ( m) m
Recent GINGER, GENESIS simulations found no significant benefit.
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3. AC-Conductivity Wakefield Effects
AC-Conductivity aspect of Resistive Wall Impedance
Was not included in calculations until 2nd half of 2004
Adds significant dependence on chamber material (Al is better then Cu)
Problem Analysis
Wakefield Theory Analysis (K. Bane) [K.L.F. Bane, G. Stupakov, “Resistive wall wakefield in the LCLS undulator beam pipe,” SLAC-PUB-10707, October 2004]
Start-To-End FEL Simulations (W. Fawley, S. Reiche)
Linear Regime Tapering Theory (Z. Huang, G. Stupakov) [Z. Huang, G. Stupakov, “Free Electron Lasers with Slowly-Varying Beam and Undulator Parameters,” SLAC-PUB-10863, December 2004]
Reflectivity Measurements
Sample Preparation (D. Walters)
Reflectivity Measurements (Jiufeng Tu, CCNY)
Low-Charge Operating Point Development (P. Emma)
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4. Mitigation of AC-Conductivity Wakefield Effects
Mitigation Strategy
Change Vacuum Pipe Properties
Change Surface Material from Copper to Aluminum
Change Cross Section from Round to Oblong (10x5 mm)
Move to Low-Charge Operating Point (200 pC)
Use Tapering in Linear Regime to Enhance Gain (200 – 300 kV/m)
Bottom Line
Goal photon intensity will be reached or exceeded
Overall pulse length shorter for 200 pC operation (~ 50%)
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5. Revisiting the Undulator Tolerance Budget
Separate budgets exist for undulator tolerances
Undulator Field Tuning/Segment Alignment/Optics Matching
BBA
Temperature Stability
Floor Stability
A Monte Carlo model is being developed which simultaneously includes all of the above errors
Calculates the cumulative phase error with MC statistics
Shows the relative importance of different tolerances
Next step is to test putative tolerance budgets against FEL code, including beam tolerances. Answer the question:
For a give overall tolerance budget, what is the probability that the FEL flux will be above 1012 photons/pulse?
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6. Undulator Segment Alignment Tolerance
Based on K Tolerance
K depends on vertical distance from mid-plane.
Canted poles make K also dependent on horizontal position
Tolerance Amplitudes
Horizontal +/- 180 microns
Vertical +/- 70 microns
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7. Cradle Component Arrangement and Alignment Problem Characterization
Two-Fold Problem for Segment Alignment
Initial installation and alignment to a straight line
Alignment maintenance in the presence of ground motion
Two Strategies under Consideration
Cradle Coupling (Train-Link)
Upstream-Downstream Beam Position Monitors
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8. 1: “Monitoring System” Train-link
Downstream quad fiducialized to undulator ends
BBA facilitates alignment of downstream cradle end and straightens electron beam
A combination of measurements using the portable stretched wire device and the portable HLS could be used to determine “loose” end offset
Monitoring System WPM and HLS provide real-time cradle position information
Info can be used as feed-back for mover system to maintain initial alignment
Before any BBA or HLS / Stretched Wire Alignment performed
Quad
Undulator Strongback
After BBA: Quad, BPM and one end of undulator aligned
Cradle
RF BPM
After HLS / Stretched Wire Alignment: Both ends of undulator aligned
Beam
R. Ruland
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9. 2: Additional Upstream Monitor
Downstream quad and upstream monitor fiducialized to undulator ends
BBA facilitates alignment of downstream cradle end and straightens electron beam
Absolute zero offset reading of upstream monitor to determine and correct “loose” end offset
Considered candidates for Upstream Monitor: 2nd RF BPM or Scan Wire
Monitoring System WPM and HLS provide real-time cradle position information
Info can be used as feed-back for mover system to maintain initial alignment
Before any BBA performed
Quad
Undulator Strongback
After BBA: Quad, BPM and one end of undulator aligned
Cradle
RF BPM
Upstream Monitor
After centering of Upstream Monitor: Both ends of undulator aligned
Beam
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10. Cradle Component Arrangement and Alignment Undulator – to – Quad Tolerance Budget
Individual contributions are added in quadrature
See R. Ruland Talk for discussion
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11. Earth Magnetic Field Effect on Trajectory
no Earth’s field – standard errors, after BBA
0.1-Gauss Earth’s field in x-direction – standard errors, after BBA
0.2-Gauss Earth’s field in x-direction – standard errors, after BBA
Paul Emma
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12. Earth Magnetic Field Compensation Strategy
Earth Magnetic Field along Beam Trajectory in Undulator requires compensation. Estimated strength 0.43±0.06 Gauss : (0.18±0.03, -0.38±0.07,0.08±0.05) Gauss based on Measurements by K. Hacker (see LCLS-TN-05-4)
Compensation Strategy:
Position the Undulator on Magnetic Measurement Bench in same direction as in Undulator Tunnel. Add correction field (Helmholtz Coils), if necessary.
Compensate Earth Field Component in Undulator in Shimming Process
Scheduling Issues :
Undulator Hall Beneficial Occupancy occurs 2 months after 33rd undulator is received.
Undulator Hall Magnetic Field can not be measured before tuning of most of the undulator segments is complete
Risk that field found in undulator hall is different from field used during shimming.
Tolerance for error field is 0.1 G.
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13. Earth Magnetic Field Compensation Adjustable Shim Concept
Risk arises from the lack of precise knowledge of the earth field in the tunnel at the time of undulator segment tuning.
Considering mitigation strategy based on use of a small number of precisely adjustable shims along each undulator.
One extra shim per segment will reduce phase error by factor 4.
Shims could be installed before undulator tuning, but adjusted before undulator installation when field errors have been determined.
Quad
BPM
Undulator
Quad
BPM
Undulator
Quad
BPM
Trajectory w/o Shim
Shim Position
Trajectory w/ Shim
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14. Undulator Commissioning Plan Overview
Pre-Beam Checkouts
Conventional Alignment
Control System Checkout
Undulator Motion Control Checkout
Magnet Polarity Checkout
Commissioning with Beam
LTU Commissioning to Tune-Up Dump
First Beam through Undulator Vacuum System (All Undulator Magnets Rolled-Out; Quads could initially be turned off)
BBA Commissioning with Undulator Magnets Rolled-Out
First Beam through Undulator Magnets
BBA Commissioning with Undulator Magnets inserted
XTOD Diagnostics Commissioning (with one or more Strongbacks inserted)
Spontaneous Radiation Characterization up to full energy and charge range
FEL Radiation Characterization at 15 Angstrom
FEL Radiation Characterization stepwise towards shorter Wavelengths
Transition to Operation
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15. Undulator Radiation Protection Considerations
Undulator Radiation Protection Greatest MP Concern during Commissioning
Sources for Undulator Radiation Damage
Upstream of Undulator
Beam Halo (Emittance, Energy Spread)  Collimators
Energy Errors  Collimators
Steering Errors  Collimators
Power Supply Failures  Collimators, Interlocks
Inside Undulator
Chamber Alignment Error  Single Shot
Steering Errors  Interlocks (SP, BPM, Radiation)
OTR Screen  Restricted Use (Automatic Monitoring and Interlocks)
Wire Scanner  Restricted Use (No Problems Expected)
Power Supply Failures  Interlocks (Radiation)
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16. FEL Measurements
Desirable measurements as function of position along undulator :
Intensity (LG, Saturation)
Spectral distribution
Bunching
Total energy
Pulse length
Spatial shape and centroid
Divergence
Saturation
Exponential Gain Regime
Undulator Regime
1 % of X-Ray Pulse
Electron Bunch Micro-Bunching
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17. Alternative: End-Of-Undulator Diagnostics
Characterize x-ray beam at single station down stream of undulator
Solid Attenuator
Gas Attenuator
Direct Imager
Indirect Imager
Spectrometer
Turn-Off Gain at Selectable Point Along Undulator by
Introduction of trajectory distortion
Roll-out of individual undulator segments
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18. Measurement of SASE Gain with Trajectory Distortion
Quadrupole Displacement at Selectable Point along Undulator
GENESIS Simulations by Z. Huang
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19. Measurement of SASE Gain Using Rollout
Undulator Segments can be removed by remote control from the end of the undulator. They will not effect radiation produced by earlier segments.
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20. Conclusions Break lengths structure simplified and finalized.
AC conductivity risk can be mitigated. (Al, Oblong Cross-Section, Gain Tapering)
Fine tuning of undulator tolerance budget is underway.
Cradle component arrangement issues are being addressed.
Mitigation for insufficient knowledge of earth field component inside undulator hall is under investigation.
The undulator commissioning plan for the LCLS is under development.
x-ray diagnostics located down-stream of undulator.
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21. End of Presentation
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